Conventional greenhouse curtains, also called shade curtains, are hung under or draped over support lines which are stretched along the length of the greenhouse near the glass roof. A relatively large number of support lines is needed, typical distances between them being about half a meter, which leads to high installation costs, long installation times, reduced illumination and a greenhouse construction strong enough to bear the load of a large number of steel wires or lines, especially when it comes to large greenhouses. For example 50.times.100 m is not unusual.
The reason that the support lines are at such a short distance from each other is that the greenhouse curtains are too weak to include support hooks in the cloth itself. Otherwise they are pulled out by wind gusts through the airing openings when they are open. Until now this problem has been alleviated by sewing a strip along the top of the curtain along the support lines, that is, along the breadth of the curtain. Another alternative is to glue a reinforcing tape there instead of the sewn on strip. There are also variants where the material has been made thicker in the area where the hooks are to be attached to better withstand external forces. None of these alternatives is optimal and all of them cost time and money and have limited strength in the environment in which they are used.
Through WO 92/12621 a greenhouse curtain is already known that is reinforced at regular intervals along its length with strips integrated in the material to which strips support hooks are attached. Because the material must not hang down between the support lines, only an unimportant sag being tolerable, the distance between the support lines cannot be increased in spite of the reinforcing strips.
Another problem is the unavoidable shrinkage of the cloth along its length even if the cloth has been shrunk during production. A greenhouse cloth can be up to 100 meters long and even if the shrinkage is as little as 0.5% it amounts to 0.5 meter which is enough to prevent the construction from working properly as the majority of the hooks that are attached to the cloth are pulled out when the shrinkage begins.
In this context it is also important that the package that is formed when the curtain is pulled aside is as narrow as possible. A large baggy package steals light and gives a bad impression.
Through SE-A-9402522-8 a plant protection construction for outdoor use is also known, that is, plant protection curtains not for greenhouses, which in contrast to greenhouse curtains must be water tight. To divert rain water the support lines are stretched between guides attached to longitudinal beams via carriers and the plant protection curtain is arranged to form sloping channels between the suspension lines. The support and the suspension lines are there mainly to prevent the formation of water pockets which can make the whole construction collapse.